All Planning Engineers, and most of project managers can create a Gantt Chart (Bar Chart) using different scheduling tools such as Primavera and Microsoft Project. However, only few of them really know how the scheduling software is calculating the network diagram in order to estimate the project duration, project floats, and critical path.

In this topic we will discuss how any software is calculating the network diagram in order to estimate project duration, float, and the critical path. Using the following figure of network diagram we will see how is the calculation works.

Each circle represent a task, each line between tasks represent the relationship. For example Task S need to be finished before task A can start, and Tasks B and C should be finished before task F can start and so on.

First of all, you need to draw a rectangular for each task as per the following figure.

ES = Early start

EF= Early finish

ID = task ID

Slack= Total Float

E= Task duration

LS= Late start

LF= Late finish

If we assumed the duration (E) for each activity as per the following:

Task

Task Duration E

A

7

B

11

C

12

D

9

E

2

F

10

G

3

H

2

I

6

J

8

In order to calculate the values is each rectangular (task), you should first fill the duration (E), calculate Early Start (ES) and Early Finish (EF). This process is called Forward Calculation EF = (ES + D) – 1

After you Finish all the tasks, from the last one you need to go back and calculate the Late Finish (LF), Late Start (LS), and the Total Float (Slack). This process is called Backward Calculation LS = (LF – D) + 1

Try to solve this network diagram by yourself, this will help you to develop a common sense when you are dealing with scheduling software such as Primavera or Microsoft Project. when you finish you will be able to find the project total duration, which will be founded on task J Early Finish or Late Finish, and you will be able to define the critical Path.

Assuming that the project duration is 30, and the critical path is ACGHI, use both of them as a password 30ACGHI to open the following link and add your name to honor board

Author

Hany Ismael BSc, MSc is a Civil Engineering post graduate student from the Liverpool University, a Project Management Professional with Certified PMP, involved in various multi level construction projects since 11 years, and currently working as Planning Manager. Hany beleives sharing knowledge helps grow and teach ourselves. Hany is rapid in updating himself of latest project management tools and techniques in Primavera and Excel.
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